Delivery systems for dispensing a beneficial agent to a biological environment of use are known to the prior art. For example, delivery devices having a fluid-permeable (semipermeable) membrane wall, an osmotically active expandable driving member and a density element for delivering a thermo-responsive beneficial agent formulation are known and are disclosed in, for example, U.S. Pat. Nos. 4,595,583; 4,612,186; 4,624,945; 4,663,148; 4,663,149; 4,675,174; 4,678,467; 4,684,524; 4,692,336; 4,704,118; 4,716,031; 4,717,566; 4,717,568; 4,717,718; 4,772,474; 4,781,714; 4,800,056; 4,814,180; 4,844,984; and 5,000,957. These dispensing devices of the prior art are extraordinarily effective for delivering beneficial agents that are hydrophilic, hydrophobic, lipophilic or lipophobic to a biological environment of use. The delivery devices operate successfully for their intended use, and they can deliver numerous difficult-to-deliver beneficial agents at a controlled and predictable rate.
However, the delivery devices, when placed into a biological environment of use, exhibit a delay to steady-state pumping or, in other words, they exhibit a startup period. One method of overcoming this startup delay is to prehydrate the device. Prehydration was found to be effective for initial test batches of the devices, producing uniform rapid startup of delivery of the beneficial agent when the devices were placed in the environment of use. But when prepared in larger-scale production lots, the devices exhibited inconsistent and much less uniform startup profiles when prehydrated with a constant volume of prehydration fluid, with many devices having an unacceptable delay before reaching steady-state pumping.
Therefore, those skilled in the art will appreciate that the development of improved delivery devices that will consistently exhibit a desired rapid startup behavior would be greatly desired and would be an advancement in the drug delivery art.